June 10, 2011 -- There are now eight new substances on the official U.S. list of toxins known to cause or suspected of causing cancer.

There are now 240 agents on the list, maintained by the National Toxicology Program (NTP). The NTP lists agents in two categories: those known to cause cancer, and those expected to be added to the "known carcinogen" list once there's more scientific evidence.

It's not possible to totally avoid exposure to carcinogens, says John Bucher, PhD, associate director of the NTP, a part of the National Institutes of Health.

"We are exposed to small levels of carcinogens every day: in drugs, in chemicals, in sunlight, in tanning beds, in tobacco smoke, over and over every day," Bucher said at a news teleconference. "This report is just to allow people to have the information they need to make choices every day. Simply avoid using products containing these substances if you are uncomfortable with the risk."

Most of the known risk comes from industrial exposures to workers at manufacturing plants. It's not clear how much risk, if any, comes from the many consumer products that emit small amounts of these carcinogens.

Bucher says he's not worried about his own daily exposures. "I probably won't be making many changes," he said.

New Carcinogens

The two new known carcinogens are aristolochic acids and formaldehyde.

Aristolochic acids are the active ingredient in a number of unsafe herbal remedies. The FDA has been warning Americans about these herbs since 2000. All herbal remedies suspected of containing aristolochic acid are banned in the U.S. and in Europe.

Formaldehyde is used to manufacture a wide range of products. The most common source of formaldehyde exposure is cigarette smoke. Cars and wood stoves give off formaldehyde, but most exposure comes from indoor air. New home finishing products and consumer goods such as some hair-smoothing/straightening products, cleaning agents, and glues may contain formaldehyde.

The six agents now "reasonably anticipated to be carcinogens" are:

Styrene, a compound used to make polystyrene. Although disposable cups made from polystyrene leach small amounts of styrene, Bucher says the amounts are very small.

Captafol, a fungicide once commonly used in agriculture but no longer produced after 1987 or used after 2006 in the U.S.

Cobalt-tungsten carbide (in powder or hard metal form) is used to make hard-metal tools. The major source of exposure is from plants manufacturing such products.

Certain inhalable glass wool fibers used in air filters or as insulation. The type of glass wool used for insulation and filtration may be less dangerous than the special kind used for manufacturing.

O-nitrotoluene is used in the manufacture of dyes. Most exposures come from air or ground pollution.

Riddelliine is a plant compound found in a type of daisy found in the Western U.S. and in other parts of the world. It has been used accidentally in medicinal herbs and may contaminate the milk of cows that graze on the plants.

Listing of a substance as a carcinogen by the NTP does not limit its use. However, NTP determinations are used by agencies such as the FDA and OSHA as the basis for regulations.